The invention relates generally to protectively sealed (plastic encapsulated) electronic circuit devices and to methods and apparatuses for forming the same. Although not limited thereto, the present invention is particularly suited to the provision of encapsulating "carriers" for integrated circuits sometimes referred to as semi-conductor "dies" or "chips" (i.e., the silicon or other semiconductive wafer itself).
In the manufacture of semi-conductor devices such as integrated circuits or semi-conductor chips, the circuitry is carried by a tiny silicon wafer or "chip". It is essential that the chip be packaged so as to protect it from the damaging effects of dust, moisture, static electricity, excessive temperature and other environmental dangers. At the same time it is necessary to provide a suitable number of electrical "leads" extending from the chip circuitry to electrical contact points external to the package. If the externally accessible leads are compliant beam leads or the like, the carrier is often said to be "leaded". If the externally accessible leads are rigidly affixed to the external carrier body, the carrier is often said to be "leadless". Such protective encapsulating structures are often referred to as chip "carriers" although other terms such as "package" or "device" have also sometimes been employed.
The two predominant types of carriers for such semi-conductor chips to date have been various types of ceramic carriers and various types of plastic encapsulated carriers. Of the two, ceramic carriers universally have been deemed to provide superior protection for integrated circuitry and have been highly preferred for critical end uses, such as space or military hardware. However, the considerable difference between thermal coefficients of expansion for ceramic and the usual epoxy-based printed circuit board (to which they typically become physically affixed) often presents a considerable reliability problem. The ceramic carrier and/or the printed circuit board undergo considerable mechanical stresses as large temperature changes are encountered--especially when so-called "leadless" chip carriers are involved (i.e., where non-resilient contact pads on the carrier are soldered to mated contact pads on the printed circuit board) as typically may be the case for ceramic carriers. In less critical applications, molded plastic carriers have been preferred because they combine much lower production cost with a lower level of protection that has been tolerated by the industry because of the savings achieved by the low cost, high volume production methods associated with the use of plastic packaging. Plastic carriers also have a thermal coefficient of expansion closely approximating that of the usual printed circuit board and are thus desirable for that reason as well.
In the present manufacture of plastic encapsulated electronic circuit devices such as integrated circuits, it is common practice to provide planar metallic lead frames initially connected together in continuous strip form. The interconnecting framework is sometimes called a "dam" because it tends to limit undesirable flow of plastic back along the leads. The strip has central chip support areas spaced along its length and a plurality of individual beam leads extending outwardly (e.g., lengthwise and transversely of the strip) from points adjacent each such support area to the outer perimeter of the strip. The beam leads are initially supported by an integral "frame" interconnecting them together in a plane as a unitary mechanical structure. With semiconductor chips thus supported, the ends of electrical wires are typically bonded to the various leads on the lead frame (usually 14 to 28 or more) and to the respective appropriate connector pads on each chip. After this has been done, the strip is then placed in a mold having a cavity for each chip and a suitable thermosetting plastic encapsulating material is forced in its viscous liquid flowable state through a mold runner system into the mold cavities. The plastic thus encapsulates the chip in each cavity, a portion of the beam leads emanating therefrom and the entirety of the bonded wire connections running between the beam leads and the chip.
After encapsulation, the leads of the lead frame are "singularized" by cutting away the "dam" portions of the integral frame structure and leaving individual beam leads extending from the sides of the plastic carrier. These beam leads are typically then bent downwardly at 90.degree. for mating with solder or other electrical connection holes or sockets in a printed circuit board. The encapsulated end products are generally referred to as packaged integrated circuits or, more simply, as "integrated circuits" or as an "IC" or even as a "chip". An example of such a plastic encapsulated IC carrier is the well known "dual-inline-package" or "DIP".
Regardless of the particular number of devices to be encapsulated at any one time, typical present day molds include pairs of heated mold members. One of the mold members is provided with a main injection opening which communicates with relatively large feed runners that extend along the lengths of the strips, with relatively small gate runners branching from the feed runners and extending to each cavity. Then, with the lead frame positioned between the mold members so that portions to be encapsulated are in registration with respective mold cavities, a suitable preheated plastic, in viscous liquid form, is injected under pressure through the main opening to fill the feed runners, gates and finally the mold cavities.
After the plastic has cured, the mold members are separated and the strips, with the encapsulated portions thereon, are removed from the mold. Ejection pins are provided on at least one of the mold members to engage the encapsulated devices and force them out of the mold cavities during mold separation. The lead frames are then "singularized" and bent for eventual mating with a printed circuit board or the like.
Although present molds and molding methods using epoxy resins generally provide an effective encapsulation for many purposes, a number of significant disadvantages do exist. For example, overall cycle time is long. Each time a mold is used, the cavities, runners and gates must be checked, cleaned and all flash removed. Otherwise, a clogged runner, gate or air vent will prevent the next molding operation from being carried out successfully. Because of the many cavities and runners, the precision machined mold members and resin distribution runners are difficult to clean and prone to damage and surface wear. Indeed, mold cleaning of the flash and cleaning of the air vents is very difficult even under the best of conditions. When using standard molding processes, the operator uses an air hose and brushes the mold surface to help remove the thin layers of cured plastic adhering to the mold surfaces. If the mold is not properly maintained the hardened surfaces become opened with thousands of tiny impressions which eventually cause, the mold to become useless as a manufacturing tool. It can be refurbished and reground but this takes the mold out of production and can take 3 days to a week or more to do.
Sometimes there is flashing in the runner areas, as well as, flash around ejector pins and cavities. This increases the amount of wasted plastic. The newer versions of these machine molds have automatic cleaning heads with rotating brushes and use air blasts or vacuum sources to remove plastic flash from the mold surfaces. Of course, it is not just the mold wear, or wasted plastic that is the main loss it is the time needed for cleaning time is money--the lost time is lost production. Cleaning usually takes at least a minute which is undesirable from a manufacturer's standpoint.
Furthermore, the epoxy resin plastic typically used to encapsulate an integrated circuit is heat curable, or thermosetting. As a consequence, much of the epoxy resin plastic which fills the runner system is wasted since, unlike thermoplastic materials which are cured by temperature reduction, thermosetting epoxy resin cannot be remelted and reused. In addition, because of the extensive runner systems of present day molds, the ratio of plastic in the runner system to the plastic actually used for encapsulation is undesirably high and results in considerable waste of plastic material
These prior encapsulation methods also pose substantial risks to the effectiveness of the protection afforded the chip circuitry. First, as the hot viscous resin is introduced under pressure into the mold cavity, the delicate wire leads from the frame to the chip are stressed by the relatively rapid flow of the hot viscous resin through and across the cavity. Upon subsequent cooling of the resin, the leads are further stressed as the resin shrinks Such stresses often result in irreparable lead damage and reduce the yield of usable components or they may induce subsequent lead failures in the field. Indeed, "infant mortality" of packaged integrated circuits has been for many years, and continues to be, a significant and costly problem to the semiconductor industry. Second, the system of runners and gates inherently produces small integral plastic appendages on each package which must be broken off flush with the outer wall of the package after the latter has been removed from its mold. The areas of fracture thus produced provide microscopic openings ("micro-cracks") into the package wall which may later serve as passageways for moisture and contaminants and which eventually may cause failure of the electronic circuit device. For example, solvents used in the cleaning of completed packages may be allowed to penetrate such micro-crack openings and cause damage.
Because of such drawbacks, a need has existed in the art for another method of plastic encapsulation. It would be of particular advantage if the method would require less plastic, create less stress on delicate leads, require less mold pressure, take less processing time, easily adapt to automation, and, most importantly, result in a chip carrier that would provide a significantly higher level of protection for the electronic device enclosed therein. It would also be advantageous to provide a method of encapsulating that reduces or eliminates the need to clean the mold following every encapsulation.
In addition, with present encapsulation systems, there is no economic and convenient way to include extra planar conductor members such as heat sink members, electrostatic shields, cross-talk shield, strengthening members or moisture barriers. Therefore, a method which permits the economic and convenient addition of such members would be of significant advantage from the standpoint of both cost and utility.
Finally, another disadvantage of conventional plastic packages, such as the DIP, is that the inherent geometry of such packages has resulted either in a limitation on the number of external leads that can be utilized or an increase in the size of the package resulting in undesirable electrical effects and package dimensions which are incompatible with the surface area requirements of the circuit board into which the package will be incorporated. It would be of great advantage to provide a plastic package which would be capable of combining high lead counts, small package size, low production cost and high volume production.